1. Field of the Invention
This invention relates to a method and apparatus for treating monofilament or multifilament material and more particularly to a method and apparatus for continuously treating a surgical suture strand.
2. Background of the Art
Surgical suture strands or threads often require post-treatment. Bioabsorbable sutures fabricated from homopolymers or copolymers of glycolide, lactide, and other bioabsorbable polymers are currently in widespread use in surgical procedures. Such sutures are often spun from multiple filaments to form a braided suture. One problem is that the finished suture often contains residual amounts of monomer, and other contaminants, which leads to a shortened in-vivo suture strength, i.e., the suture absorption rate is too rapid. Monomer residue can result from incomplete polymerization. Also, the spinning process can form monomer.
One way of removing the monomer and other vaporizable contaminants, is to heat the suture material in a vacuum and/or under the flow of a dry nitrogen atmosphere. Presently, suture post-treatment is accomplished by a static batch process in which the finished suture is wound around a spool, and the spool is placed in a heating chamber under the appropriate atmosphere. One such method is exemplified in U.S. Pat. No. 3,772,420 to Glick, which discloses a method for improving the in-vivo strength of polyglycolic acid braid by wrapping the braid around a cylinder and placing the braid and cylinder in a vacuum chamber held at from 100.degree. to about 150.degree. for a period of time of from about 1 to 3 hours. This method possess the disadvantage of uneven removal of monomer. The monomer from the threads near the exterior of the spool is easily removed whereas the monomer from inside the spool of suture thread must travel through many more layers of suture to be removed.
In addition to modification of the suture composition, suture post-treatment can often be required for mechanical modification such as stretching or relaxing a thread. Sutures of monofilament polypropylene, for example, are relaxed to decrease stiffness, and to increase elongation at breaking point. Up to now, batch processes have been used for polypropylene filament relaxation.